TFII-II is an important multi-functional transcription factor that links events to transcription in several genes. TFII-I is constitutively associated with Bruton's tyrosine kinase (Btk), a non-receptor tyrosine kinase that is essential for normal B cell function, as its mutation causes X-linked agammaglobulinemia (XLA) in humans and X-linked immune deficiency (xid) in mice. We propose that TFII-I is an important and novel component in linking Btk-mediated signaling to transcription in B cells. Furthermore, the TFII-I gene gets deleted in William's syndrome (WS) which is a neuro-developmental disorder with multi-system manifestations, including supravalvar aortic stenosis, hypercalcemia in infancy, mental retardation and cognitive defects. Thus, TFII-I appears to be involved in two genetic disorders: William's Syndrome and X-linked agammaglobulinemia (XLA). Knowledge gained from these studies may help us better understand a critical Btk dependent pathway that links B cell receptor mediated signal transduction to B cell specific transcription. These studies may also ultimately help identify potential target gene(s) that are affected by mutations in Btk. Importantly, these studies may establish possible connections between the neuro-developmental disorders (as in WS) and immuno-developmental disorders (as in XLA). Toward a better understanding of TFII-I function in Btk mediated immune response, we will first map the region(s) in TFII-I important for its physical and functional interactions with BTK. We will determine by deletion and point mutation the region(s) in TFII-I that is important for its interaction with Btk, followed by mapping the sites in TFII-I that are tyrosine phosphorylated by Btk in vitro and in vivo by a combination of site directed mutagenesis, phosphopeptide, finger printing, and mass spectrometric analysis. We will also analyze these mutants in functional transient transfection assays. To determine the functions of TFII-I and its biochemical interactions with Btk in B cells, we will employ in vivo transcriptional analysis. To determine the functions of TFII-I and its biochemical interactions with Btk in B cells, we will employ in vivo transcriptional analysis followed by the interaction studies by co- immunoprecipitation and ectopic expression of mutant forms of TFII-I in B cells. We will also stably express wild type and mutant forms of TFII-I, and Btk in B cell lines, and genetically delete TFII-I from chicken B cells. Finally, to ascertain the localization of TFII-I in the absence and in the presence of non-activated versus activated Btk, first, we will co-express various mutants of TFII-I with Btk in COS cells. Subsequently, we will employ freshly isolated primary splenic B cells derived from wild type, xid and Btk-/- mice and study the localization and tyrosine phosphorylation of TFII-I in the absence and in presence of B cell receptor signaling.